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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.allocation"></a><a class="link" href="allocation.html" title="Custom Memory Allocation">Custom Memory
        Allocation</a>
</h4></div></div></div>
<p>
          Many asynchronous operations need to allocate an object to store state
          associated with the operation. For example, a Win32 implementation needs
          <code class="computeroutput"><span class="identifier">OVERLAPPED</span></code>-derived objects
          to pass to Win32 API functions.
        </p>
<p>
          Furthermore, programs typically contain easily identifiable chains of asynchronous
          operations. A half duplex protocol implementation (e.g. an HTTP server)
          would have a single chain of operations per client (receives followed by
          sends). A full duplex protocol implementation would have two chains executing
          in parallel. Programs should be able to leverage this knowledge to reuse
          memory for all asynchronous operations in a chain.
        </p>
<p>
          Given a copy of a user-defined <code class="computeroutput"><span class="identifier">Handler</span></code>
          object <code class="computeroutput"><span class="identifier">h</span></code>, if the implementation
          needs to allocate memory associated with that handler it will obtain an
          allocator using the <code class="computeroutput"><span class="identifier">get_associated_allocator</span></code>
          function. For example:
        </p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">associated_allocator_t</span><span class="special">&lt;</span><span class="identifier">Handler</span><span class="special">&gt;</span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">get_associated_allocator</span><span class="special">(</span><span class="identifier">h</span><span class="special">);</span>
</pre>
<p>
          The associated allocator must satisfy the standard Allocator requirements.
        </p>
<p>
          By default, handlers use the standard allocator (which is implemented in
          terms of <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span>
          <span class="keyword">new</span><span class="special">()</span></code>
          and <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span>
          <span class="keyword">delete</span><span class="special">()</span></code>).
          The allocator may be customised for a particular handler type by specifying
          a nested type <code class="computeroutput"><span class="identifier">allocator_type</span></code>
          and member function <code class="computeroutput"><span class="identifier">get_allocator</span><span class="special">()</span></code>:
        </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">my_handler</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
  <span class="comment">// Custom implementation of Allocator type requirements.</span>
  <span class="keyword">typedef</span> <span class="identifier">my_allocator</span> <span class="identifier">allocator_type</span><span class="special">;</span>

  <span class="comment">// Return a custom allocator implementation.</span>
  <span class="identifier">allocator_type</span> <span class="identifier">get_allocator</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span>
  <span class="special">{</span>
    <span class="keyword">return</span> <span class="identifier">my_allocator</span><span class="special">();</span>
  <span class="special">}</span>

  <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
          In more complex cases, the <code class="computeroutput"><span class="identifier">associated_allocator</span></code>
          template may be partially specialised directly:
        </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">asio</span> <span class="special">{</span>

  <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Allocator</span><span class="special">&gt;</span>
  <span class="keyword">struct</span> <span class="identifier">associated_allocator</span><span class="special">&lt;</span><span class="identifier">my_handler</span><span class="special">,</span> <span class="identifier">Allocator</span><span class="special">&gt;</span>
  <span class="special">{</span>
    <span class="comment">// Custom implementation of Allocator type requirements.</span>
    <span class="keyword">typedef</span> <span class="identifier">my_allocator</span> <span class="identifier">type</span><span class="special">;</span>

    <span class="comment">// Return a custom allocator implementation.</span>
    <span class="keyword">static</span> <span class="identifier">type</span> <span class="identifier">get</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">my_handler</span><span class="special">&amp;,</span>
        <span class="keyword">const</span> <span class="identifier">Allocator</span><span class="special">&amp;</span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">Allocator</span><span class="special">())</span> <span class="keyword">noexcept</span>
    <span class="special">{</span>
      <span class="keyword">return</span> <span class="identifier">my_allocator</span><span class="special">();</span>
    <span class="special">}</span>
  <span class="special">};</span>

<span class="special">}</span> <span class="special">}</span> <span class="comment">// namespace boost::asio</span>
</pre>
<p>
          The implementation guarantees that the deallocation will occur before the
          associated handler is invoked, which means the memory is ready to be reused
          for any new asynchronous operations started by the handler.
        </p>
<p>
          The custom memory allocation functions may be called from any user-created
          thread that is calling an Boost.Asio library function. The implementation
          guarantees, for the asynchronous operations included with the library,
          that within the context of an individual operation the implementation will
          not make concurrent calls to the memory allocation functions for that handler.
          The implementation will insert appropriate memory barriers to ensure correct
          memory visibility should an asynchronous operation need to call the allocation
          functions from different threads. (Note: If the same allocator is shared
          across multiple concurrent asynchronous operations, this can result in
          concurrent calls to the memory allocation functions. Use of a <a class="link" href="strands.html" title="Strands: Use Threads Without Explicit Locking">strand</a>
          does not prevent these concurrent calls, as an operation may need to allocate
          memory from outside the strand. In this case, the shared allocator is responsible
          for providing the necessary thread safety guarantees.)
        </p>
<h6>
<a name="boost_asio.overview.core.allocation.h0"></a>
          <span class="phrase"><a name="boost_asio.overview.core.allocation.see_also"></a></span><a class="link" href="allocation.html#boost_asio.overview.core.allocation.see_also">See
          Also</a>
        </h6>
<p>
          <a class="link" href="../../reference/associated_allocator.html" title="associated_allocator">associated_allocator</a>,
          <a class="link" href="../../reference/get_associated_allocator.html" title="get_associated_allocator">get_associated_allocator</a>,
          <a class="link" href="../../examples/cpp11_examples.html#boost_asio.examples.cpp11_examples.allocation">custom memory
          allocation example (C++11)</a>.
        </p>
</div>
<div class="copyright-footer">Copyright © 2003-2024 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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